Mass spectrometers



Aug. 8, 1961 R. D, CRMG 2,995,658 I MASS SPECTROMETERS Filed Jan. 19,1960 2 Sheets-Sheet -1 INVENTOR ROBERT DEREK CRAIG gin- 3 1961 R50.CRAIG 2,995,658

mss SPECTROMETERS Filed Jan. 19, 1960 2 Sheets-Shoat 2 INVENTOR ROBERTDEREK CRAIG Eda-1 w Patented Aug. 8, 1961 fine 23%,658 MASSSPECTRGMETERS Robert Derek Craig, Altrincham, England, assignor toAssociated Electrical Industries Limited, London, England, a company ofGreat Britain I Filed Jan. 19, 1960, Ser. No. 3,426 Claims priority,application Great Britain Jan. 19, 1959 2 Claims. (Cl. 250-413) Thepresent invention relates to mass spectrometers and more particularly toion sources for mass spectrometers.

A mass spectrometer is an apparatus which can be adapted for analysingthe constituent elements of a material. The material is ionized and thismay, for example, be performed by causing an intermittent arc to formbetween two rod electrodes formed from the material and situated invacuo. The resultant ions formed from the different constituent elementsof the material will have different masses dependent on the position ofthe elements in the periodic table. Those ions with a positive chargeare formed into a beam by accelerating and focusing electrodes and anarrow section of this beam is selected by an aperture known as thesource slit. This narrow section of the beam is subsequently passedthrough an analysing field, or fields, where the ions are deflectedlaterally according to their mass and the field strength.

When an ion beam comprising ions of different mass passes through thisanalyser region itwill therefore be dispersed and a mass spectrum willbe formed. By collecting the dispersed beam on a suitable detector,which may be a photographic plate, an indication may be ob tained of theconstituent elements of the material and the quantity of each elementpresent in the material.

It will be appreciated that the analyser region may simply comprise auniform magnetic field alone or a nonuniform magnetic field alone orsome combination of electric and magnetic fields as is well known in theart.

The material to be analysed is normally located in a chamber called anion source. In this chamber, which is evacuated, the ions produced fromthe material are formed into a beam by the accelerating electrodes andare' focussed so as to pass through the narrow source slit aperture.This aperture connects with the main body of the spectrometer whichcontains the analyser. If this small aperture is the only connectionbetween the source and analyser regions and if separate pumping of thesetwo regions is provided it is then possible to maintain a large pressuredifferential between the source and analyser regions under high vacuumconditions. This is desirable since for many problems the best possibleanalyser vacuum is required While the source vacuum may not be critical.

If it is required to change the material of the specimen, it may benecessary to open up the source, remove an existing specimen and replaceit with a second specimen. If the ion source is connected to the mainbody of the spectrometer, this operation will destroy the vacuum in themain body and it will be necessary to restore the vacuum throughout thespectrometer every time the specimen is changed. This is undesirablesince the restoration of the vacuum throughout the spectrometer takes along time and it is therefore desirable to be able to isolate the mainbody of the mass spectrometer from atmospheric pressure in the ionsource. If this is done only the vacuum in the ion source will bedestroyed and since the volume of the ion source is small and because ofthe differential pumping described above the vacuum therein may berestored to a satisfactory level in a short time.

It is not however convenient to use normal types of valve for thispurpose. A valve with an axially moving closing member is unsuitablebecause when such a valve is open it does not leave an uninterruptedpath for the ion beam. A valve with a laterally moving closing member isalso unsuitable since the use of a high vacuum necessitates veryefiicient sealing and this cannot easily be obtained with this type ofvalve.

The object of the present invention is to provide an ion source for amass spectrometer having an improved type of valve adapted to seal theion source from the main body of the mass spectrometer.

According to the present invention, an ion source for a massspectrometer comprises a chamber, means for producing ions within saidchamber, a duct extending from said chamber and adapted to be connectedto the main body of the mass spectrometer, means for projecting a beamof ions from the chamber along said duct, a valve plunger movable in adirection inclined obliquely to the axis of said duct, a seating for oneend of said valve plunger extending obliquely across said duct andperpendicularly to the direction of movement of said valve plunger andmeans for moving said plunger so that it may engage in a gas-tightmanner with said seating and thereby form a gas-tight seal in said duct,and means for moving said plunger away from said seating so that it doesnot obstruct said duct.

The ion source also comprises a removable cover plate which will enablethe specimens from which the ions are produced to be removed from thesource and to be replaced with a different specimen when the valveplunger is in its closed position. This enables the specimen to bechanged without destroying the vacuum in the main body of the massspectrometer.

In order that the invention may be more readily understood, referencewill now be made to the accompanying drawings in which:

FIG. 1 is a side view, sectioned on a plane through the axis of an ionsource,

FIG. 2 is an end view of the ion source with the end cover plate removedand FIG. 3 is a diagrammatic view of a mass spectrometer.

With reference to these three figures, the main body of the ion sourceis formed from a cylindrical metal drum 1 closed at one end by a fixedblock 2 and closed at the other end by a removable end plate 3. Thisdrum is conveniently formed from a stainless steel block. The block 2 isformed with a duct 4 which extends axially from within the ion source tothe outer face 5 of the block 2. The material from which an ion beam isto be produced is formed into two thin rod electrodes 6, 7 which arepositioned so that they extend radially across the inside of the ionsource. Their ends are separated by a small gap which is aligned withthe inner end of the duct 4. The two rod electrodes are respectivelyheld in two clamps 11, 12 which in turn are respectively mounted on twoinsulators 13, 14. These two insulators are attached to the ends of rods15, 16 which extend respectively through apertures 17, 18 in thecylindrical walls of the ion source and are sealed therein by flexiblebellows 19, 21. Two devices, denoted by 22, 23, and each including threeadjustable screws 75, 76, 77, which are capable of adjustment in threemutually perpendicular directions enable the rods 15, 16 to be movedrelatively to one another so that the size and position of the gap 8relative to the duct 4 can be varied. In normal operation, the twoelectrodes 6, 7 are aligned and extend radially across the ion source. Ahigh voltage bushing 26 extends from the outside of the ion source andcomprises a metal tube 27 and a glass insulating tube 23. One highvoltage terminal 29 extends from the glass tube into an aperture 31 inthe Wall of the ion source, and a lead 32 connects the ends of thisterminal to the rod electrode 6. A second high voltage terminal 33 alsoextends from the glass tube through the aperture 31 and is connected torod electrode 7 by a second lead 34. The two high voltage terminals areexternally connected to suitable sources of electric potential.Conveniently, the terminal 33 is connected to a source of 20 kv., D.C.,and the terminals 33 and 29 are connected across an intermittent sourceof 100 kv., AC. The glass tube is sealed from the inside of the ionsource and may be evacuated separately.

Between the gap 8 and the inner face 37 of the block 2 there is locateda metal plate 38, which is connected to the lead 34, and an acceleratingand focusing electrode 39. The plate 38 is formed with a centralaperture 41 which is covered by a removable disc 42 with a centralaperture 43. Both the plate 38 and the disc 42 are conveniently made oftantalum. The plate 38 is supported by insulators 40 from the face 37 ofthe block 2. The accelerating and focusing electrode has a centralaperture. A slit 30 is formed at the end of the duct 4 in the surface37. This slit is narrow and conveniently measures approximately .002inch by .040 inch. On the end face of the block 2 is mounted anelectrically controlled shutter. This shutter is located over the end ofthe aperture 4 and is formed from three apertured conducting plates 44,45, 46, mounted in parallel planes and insulated from one another. Thetwo outer plates 44, 46 are earthed and the middle plate 45 is dividedinto two halves. One half is earthed and the other half may be connectedeither to earth or to a source of 2 kv. by a lead 47 through a suitableswitch.

A duct 51 extends from the inner end 52 of the aperture 4 towards theoutside of the block 2. This duct extends at an angle to the axis of theduct 4. A plunger 53 is adapted to move in this duct and at the innerend of the plunger is mounted a hardened steel ball 54 which is arrangedto bear on a stainless steel seating 55. The seating extends across theduct 4. The outer end of the plunger 53 passes through an aperture 56 ina sealing block 57 and bellows 58 seal the plunger within the aperture.The plunger extends within a cylinder 61 and .terminates in a piston 62.Fluid from a reservoir 63 may be passed into the cylinder 61 so as tomove the plunger either inwardly or outwardly. This operation eitherpresses the ball 54 on to its seating or else moves the ball clear ofthe end 52 of the duct 4.

Two tubes 65, 66 extend inwardly from the outside of the ion sourcetowards electrodes 6, 7. The ends of the two tubes are closed with twoglass bowls 67, 68 and these two bowls, which have surfaces which areeasily cleaned, enable the inside of the ion source to be inspected,when the end plate 3 is in position, with the aid of a suitable lightsource viewing means 78.

The end face 5 is sealed on to the main body portion 69 of the massspectrometer. The end plate 3 is bolted or otherwise attached to thebody 1 of the ion source and is sealed thereto by means of a rubber ring71 located in an annular groove 72 in the surface of the plate.

The ion source may be evacuated through a duct 73 which is connected toa suitable pump 74. The main body portion 69 of the mass spectrometerwill also be evacuated by a suitably connected pump. The only eonnectionbetween the ion source and the main body portion is through the duct 4.The ball valve formed by the ball 54 and the seat 55 will seal off thisaperture and will allow the vacuum in the ion source to be destroyedwithout afiecting that in the main body of the spectrometer. Hence whenthe valve is closed, the end plate 3 may be removed and the specimenelectrodes 6, 7 may be replaced without destroying the vacuum in themain body of the mass spectrometer. When the electrodes 6, 7 arereplaced it is possible also to replace the removable disc 42.

In operation, the electrodes are located in position within the ionsource and when the end plate 3 is sealed in position the ion source isevacuated. The valve is opened and suitable potentials are applied tothe electrodes 6, 7 and an intermittent arc is formed between theadjacent ends of these electrodes. This are produces ions of theconstituent elements of the material of the electrodes. These ions areaccelerated and focussed by the action of the plates 38 and 39 and arepassed through the aperture 43. The beam then passes through the slitaperture 30 and the shape of this aperture controls the size of thebeam. The beam passes into the duct 4 and if the ball 54 is retractedoff the seat 55 the ion beam will pass through the block 2 into theelectrically controlled shutter. If one half of the middle plate 45 ofthis shutter is connected to the 2 kv. supply and the other half of theplate is earthed, the ion beam will be deflected and will not passthrough the aperture in the outer plate 46. If, however, both halves ofthe plate 45 are earthed the beam will pass through the aperture in theouter plate 46 into the main body of the mass spectrometer. The ion beamwill then pass into an analyser of any of the well known types. The ionbeam is controlled by the potentials on the electrodes and theelectrodes must first be accurately lined up so that the ion beam willpass through the aperture 43. The plate 42 will be contaminated by strayions and neutral particles and it may be necessary to replace this platewhen the material of the electrodes is also changed.

The vacuum in the analyser region is lower than that in the ion sourceand the slit 30 etiectively isolates the source from the duct 4 in sofar as the vacuum is concerned. When it is required to change thespecimen electrodes the ball valve is closed, the vacuum is releasedfrom the ion source, the end plate 3 is unsealed and removed, and theelectrodes and the plate 42 are replaced. The end plate 3 is then scaledback on to the ion source and the vacuum is restored. It will beappreciated that a certain amount of gas at the source region pressurewill be trapped between the slit 30 and the seating of the ball valveand when the ball valve is subsequently opened, this gas will pass intothe analyser region. This space is therefore kept as small as possible,and therefore it is very convenient to have the ball valve acting at anangle to the axis of the aperture.

The plunger 53 has been described above as being moved by a hydraulicsystem. Alternatively the plunger could be removed by a mechanicalsystem, or manually.

The invention is not confined to the type of ion production described,and any other well known method of production of ions may be used.

There is described above an ion source for a mass spectrometer whichcomprises a valve which enables the ions source to be sealed oil fromthe main body of the mass spectrometer so that the specimen material maybe changed without destroying the vacuum in the main body of the massspectrometer.

FIG. 3 illustrates diagrammatically a mass spectrometer including an ionsource 81 which may be of the type described above, and from which isprojected a beam of ions 82. This beam of ions is passed through a slit83 in the end wall of the source 81 and passes into the magnetic held 84of a magnetic analyser. The beam is dis persed in this magnetic fieldinto a spectrum and is received by a collector 85 through a slit 86. Anamplifier 87 receives signals from the collector and gives an indicationof the quantityof ions of each mass which are present in the materialbeing analysed.

What I claim is:

1. An ion source for a mass spectrometer comprising a cylindrical metaldrum forming a chamber, means for producing ions within said chamber, ablock including surfaces defining a duct extending from said chamber andadapted to be connected to the main body of the mass spectrometer, meansfor projecting a beam of ions from said chamber along said duct, a valveplunger movable in a direction inclined obliquely to the axisof saidduct, a hardened steel ball on the end of the valve plunger, surfaces tosaid block defining a stainless steel seating for said steel ballextending obliquely across said duct and perpendicularly to thedirection of move ment of said valve plunger, means for moving saidplunger so that it may engage in a gas tight manner with ssid seatingand thereby form a gas tight seal across said duct, and means for movingsaid plunger away from said seating so that it does not obstruct saidduct.

2. An ion source for a mass spectrometer comprising a cylindrical metaldrum forming a chamber, a block including surfaces defining a ductextending from said chamber and adapted to be connected to the main bodyof the mass spectrometer, two rod electrodes extending within saidchamber in two opposite directions substantislly perpendicular to theaxis of said duct, means for forming an intermittent are between theadjacent ends 1 of said electrodes so as to produce ions, means forproiecting a beam of said ions from said chamber along said duct, avalve plunger movable in a direction inclined obliquely to the axis'ofsaid duct, surfaces to said block defining a seating for one end of saidplunger extending obliquely across said duct and perpendicularly to thedirection of movement of said valve plunger, means for moving saidplunger so that it may engage in a gas tight manner with said seatingand thereby form a gas tight seal across said duct, and means for movingsaid plunger away from said seating so that it does not obstruct saidduct.

References Cited in the file of this patent UNITED STATES PATENTS2,852,683 Peters et a1. Sept. 16, 195s

